Duct wall acoustic treatment

ABSTRACT

A duct wall is provided with acoustic sound suppressing treatment comprising a panel including a plurality of cavities. The cavities are of various volumes and are intermixed according to volume within the panel along the duct wall. Each cavity is connected to the duct by means of a neck passage which provides substantially direct communication therebetween. Preselected of the cavities are spaced from the duct by predetermined nonuniform distances so that a variety of frequency bands are suppressed. Generally, larger volume cavities are spaced from the duct by greater distances and are associated with longer neck passages.

United States Patent 1 1 Motsinger June 25, 1974 41 DUCT WALL ACOUSTICTREATMENT 3,726,359 4/1973 Dierl et a1 181/33 G X [75] Inventor:RussellE. Motsinger, Cincinnati, OREIGN PATENTS 0R APPLICATIONS Ohlo348,808 10/1960 Switzerland 181/33 G Assignee: General Electric Company,Great Brltam G Cmcmnatl Ohlo Primary ExaminerRichard B. Wilkinson [22]F1led: Feb. 1, 1973 Assistant Examiner-John F. Gonzales [21] APPL328,713 iilgtzgzfgAgent, 0r FirmJames M. Kipling; Derek P.

[52] US. Cl 181/33 G, 161/68, 161/139, [57] ABSTRACT Int Cl 1 3 A ductwall is provided with acoustic sound suppress- I o a s 6 6 s v s s 1 s aa a 6 6 s I a a [58] F of Search 181/33 33 33 of cavities. The cavitiesare of various volumes and are 181/33 l6l/68 139 intermixed according tovolume within the panel along the duct wall. Each cavity is connected tothe duct by [56] References C'ted means of a neck passage which providessubstantially UNITED STATES PATENTS direct communication therebetween.Preselected of 2,887,173 5/1959 Boschi 181/33 G UX the cavities arespaced from the duct by predeter- 3,159,236 12/1964 Akerson 181/33 G UXmined non-uniform distances so that a variety of fre- 3,l66,l49 l/l965Hulseet al 181/33 G UX queney bands are suppressed, Generally, largervolg 'ume cavities are spaced from the duct by greater disary e a3,353,626 11/1967 Cremer et a1 181/48 x tances and are assoclated longerneck passages 3,688,866 9/1972 Kong 181/33 K 5 Claims, 3 Drawing Figures.54 4 J34 Q 1 c 7 LL! 1 1 I U I In L 11 1 j DUCT WALL ACOUSTIC TREATMENTBACKGROUND OF THE INVENTION This invention relates to sound suppressionand, more particularly, to acoustic treatment of duct walls in gasturbine engines.

The invention herein described was made in the course of or under acontract, or a subcontract thereunder, with the United States Departmentof the Army.

Various means have been used in the past to suppress noise generatedwithin gas turbine engines. Generally, noise treatment has been directedprimarily toward high frequency noise, and one form of this treatmenthas utilized the well known Helmholtz resonator disposed in pluralitieswithin a duct wall treatment of the engine.- Generally, these devicescomprise a plurality of cavities connected to the duct by means ofapertures which form entrances for generated sound waves.

Each individual cavity is capable of attenuating sound wave propagationwithin discrete frequency bands by means of energy dissipation withinthe cavity resulting from pressure losses of the flow passing in and outof the cavities. The frequency bands-within which an individual cavityis effective are closely limited about a resonant frequency which isestablished by the Helmholtz resonator equation:

W resonant frequency, H

C speed of sound in the medium, feet per second;

S area of the resonator neck, feet squared;

l the effective length of the resonator neck, including an endcorrection, feet; and

V contained volume in the resonator cavity, feet cubed.

In order to effectively remove objectionable sound phenomena from gasturbine engines, it is desired to treat sound of both high and lowfrequencies. Prior attempted solutions, not based upon Helmholtzresonators, have included the utilization of a thin porous layer over asubdivided air space. In another attempt, a solid blanket ofporousmaterial has been used to treatduct walls. However, both of theseattempts have suffered from the fact that each material suppresses soundas a function of the material thickness, so that low frequencyattenuation requires high thickness. Particularly with respect to gasturbine engines used to power jet aircraft, overall nacelle thickness isof prime importance with respect to drag performance. Hence, soundattenuation based upon increasing duct wall thickness is undesirable.

Alternatively, duct walls have been treated with discrete layers ofdifferent sized Helmholtz cavities separated from the duct and from oneanother by thin porous face sheets so that sound of higher frequency isretained within the duct-bordering smaller cavities, while lowerfrequency sound penetrates these smaller cavities and is dissipatedwithin radially outward larger cavities. However, once again, thisapproach suffers from affects on overall duct thickness.

Still another approach has been to align axially several panels ofHelmholtz resonator cavities, each panel housing cavities of differentsize. This treatment has efficiently lessened sound propagation from theduct, but

at the substantial negative affect of requiring extension of the ductlength. Duct length is another parameter which is preferably minimizedin most gas turbine engines. Hence, this treatment also has met withlimited usefulness.

The present invention deals successfully with the contemporaneousproblems of wide frequency range sound suppression, duct lengthminimization, and nacelle thickness minimization. The basic conceptsinvolved can be elucidated with reference to the foregoing Helmholtzresonatorequation. Referring to the denominator of the radial function,it can be seen that resonator frequency (W is inversely proportional tothe product of the effective length (l') of the resonator neck and thecontained volume (V) in the resonator. Hence, elongation of the neckpassage permits reduction of the cavity volume without affectingresonant frequency. The present invention makes use of this concept bydisposing a plurality of neck passages within an acoustic panel in sucha way as to provide a plurality of relatively large volume cavities forthe suppression of low frequency sound along with smaller volumecavities without the necessity of overly lengthening or thickening thepanel itself. This is accomplished, in general, by utilizing structurealready present in the panel to form various cavities and neck passages.

Stated in another way, the present concept permits combining severaldifferent size cavities into a configuration which inherently provides avariety of neck lengths. Thus, this invention provides for the use ofseveral different effective frequency suppression bands without thenecessity of adding substantially to the length or thickness of the ductwhich must be treated.

BRIEF DESCRIPTION OF THE INVENTION Consequently, it is a primary objectof the present invention to provide for improved suppression of a widerange of noise frequencies, including low frequency noise, by theutilization of duct acoustic treatment which does not requiresubstantial enlargement either in the length of duct treated orthickness of treatment.

Stated briefly, the present invention performs this and other objectivesby the utilization of a sound suppressing panel comprising a pluralityof cavities having a variety of cavity volumes and spaced from the ductwithin the panel by a variety of distances. A plurality of interspersed,various length neck passages is provided for providing substantiallydirect communication between the various cavities and the duct. The neckpassages vary in length depending upon the disposition of the associatedcavities within the panel, and their separation from the duct. In orderto save weight as well as to minimize thickness and length, wallportions defining smaller cavities also combine to form the neckpassages for larger cavities. In this way, a pyramiding effect isrealized whereby a broad spectrum of sound frequencies may be absorbedand dissipated within the single sound suppressing panel.

BRIEF DESCRIPTION OF THE DRAWING The present invention may be moreclearly understood by reference to the appended specification as well asthe drawings wherein:

FIG. 1' is a cross-sectional view of a simplified gas turbine engine;

FIG. 2 is an enlarged cross-sectional view of a portion of the treatedduct wall of FIG. 1 according to the present invention; and

FIG. 3 is an enlarged cross-sectional view, similar to that in FIG. 2,but representing another embodiment of the present invention.

DETAILED DESCRIPTION In FIG. 1, the simplified gas turbine enginedesignated generally includes an annular duct inlet 12, a bladed fan 14,a compressor 16, combustion chambers 18, and turbine along with exhaustnozzle .22. This engine operates in a fashion similar to typical enginesof this variety. Atmospheric air enters inlet 12 to the left in FIG. 1tobe operated upon and compressed by fan 14 and compressor 16, a portionof the flow passing through a fan duct 24 and the remainder through thecompressor 16 and into conbustors 18. Within the combustors, the air ismixed with fuel and ignited, whereupon rapid expansion of the fueloccurs and a high velocity stream of products of combustion exits thecombustors to the right and engages the rotatable bladed stages ofturbine 20 to impart torque thereto for the operation of fan 14 andcompressor 16. The gas stream exiting the turbine 20 is combined withthe fan stream within fan duct 24 and expelled through the exhaustnozzle 22 to provide a substantial thrust toward the left in the Figure.v Objectionable engine noise has been determined to be generated inseveral portions of the engine: the fan blades rotating at high tipvelocities generate a broad spectrum of noise frequencies; compressorsand turbines generally high'frequency noise; and the combustors are alsoa noise source. As a means for reducing the noise propagated from theengine, the present invention provides a sound suppressing treatment forthe wall 26 defining the duct 12 (or for similar duct surfacesthroughout the engine).

More specifically, referring to FIG. 2, a first embodiment of thepresent invention is depicted in application to wall 26, as an exampleof typical wall applications. This figure shows an enlarged view of theduct wall 26 whereupon a panel 27 incorporating a plurality of Helmholtzresonator type cavities is disposed. Small cavities 30 having a firstpredetermined volume and disposed immediately adjacent the duct 12 arepartially defined by a number of radially extending side walls 32 and anaxially extending backing wall 34. Each of the first cavities 30 isseparated from the duct by means of the thickness of axially extendingduct wall 26, and in which a plurality of apertures 36 are disposed.Each of the small cavities 30 opens directly into the duct 12 by meansof one of the apertures 36. (More than one aperture can serve eachcavity, but for the sake of simplicity, a one-for-one relationship ismaintained throughout this discussion.) A second plurality of cavities40 is also incorporated into the panel 27. The cavities 40 have a secondpredetermined volume larger than the volume of cavities 30. Each cavity40 is partially defined by an axially extending wall 44, along withwalls 34 (which also bounds cavities 30, as stated). Moreover, it can beappreciated from the figure that the previously mentioned walls 32 andradially extending walls 42 (by which cavities 40 are further defined)are common walls to the extent of the length of walls 32. Such doubleusage of walls results in substantial weight savings due to the absenceof a requirement for adding redundant walls.

Each cavity 40 is connected to duct 12 by means of one (or more, asabove) radially extending neck passage 46 providing substantially-directcommunication between the cavity 40 and the duct 12. It can be seen fromthe figure that neck passages 46 are defined by opposed pairs of thewalls 32 of adjacent first cavities 30. Hence, a further weight savings.

The interrelationship between the first and second plurality of cavities30 and 40, respectively, is such that cavities 30 are disposed generallyin a layer (owing to axial colinearity of walls 34) which is disposed tothe radial interior of a second layer of cavities 40 (produced by asimilar colinearity of walls 44). Each cavity 40, in this firstembodiment, substantially circumscribes at least one pair of cavities30. Moreover, the cavities 30 lie substantially between the duct andcavities 40.

Proper functionality of the cavities as Helmholtz resonators requiresthat there be an entrance to each cavity for sound waves. The entrancewith respect to each of cavities 30 is an aperture 36. The entrances to.cavities 40 are neck passages 46. Axially, it can be seen that the firstand second neck passages 36 and 46, respectively, are substantiallyinterspersed with respect to one another so that sound waves occurringwithin a given length of duct 12 will encounter both varities of neckpassages and enter both types of cavities.

The functional importance of this particular configuration will now bedescribed. As previously stated, the gas turbine engine noise emanatingforward through duct 12 comprises a plurality of frequencies.Furthermore, the sound dissipation capacities of individual Helmholtzresonator type cavities are narrowly limited about predeterminedresonant frequencies, determined, in part, by the product (lV) of cavityvolume and neck passage length. As sound waves of various frequenciesadvance forward through duct 12 and encounter the plurality ofinterspersed neck passages 36 and 46, portions of the sound waves andassociated sound energy enter each neck passage. Higher frequency waveswill be effectively dissipated within cavities 30. Lower frequency waveswill be dissipated within cavities 40; Consequently, the configurationas thus far described will effectively dissipate sound energy in bandscentered about two discrete resonant frequencies, one characteristic ofeach combination of cavities (30, 40) and neck passages (36, 46)described.

To further enhance the sound dissipation capability of the embodimentdepicted in FIG. 2, a third plurality of cavities 50 are defined by anumber of radially extending walls 52 and an axially extending wall 54along with a second weight-saving usage of walls 44. A neck passage 56is defined by side walls 42 of adjacent cavities 40. The volume ofcavities 50 is larger than the volume of cavities 40 (which is in turnlarger than the volume of cavities 30). Furthermore, the length of neckpassages 56 is.longer than, that of neck passages 46 (which isin turnlonger than that of neck passages 36). Hence, the product lV is largestwith respect to cavities 50, smaller with respect to cavities 40 andsmallest with respect to cavities 30. Moreover, the entrances to neckpassages 36, 46 and 56, are all interpersed with one another. As aconsequence, the acoustic panal of the present invention, according tothis first embodineck passage locations, the present invention makespossible the treatment of wide ranges of sound frequencies along asingle length of acoustic panel without the necessity for addingadditional lengths. Contemporaneously, the utilization of the cavitywalls within the panel 27, to both define individual cavities as well asthe various walls and neck passages for other cavities, results in alightweight configuration. Moreover, the fact that the neck passagelength for the lower frequency cavities are relatively long results inan ability to treat low sound frequencies with relatively small cavityvolumes. (This fact can be appreciated by recalling that resonantfrequency is a function of the product of neck passage length and cavityvolume). Hence the overall panel thickness is advantageously minimized.

In general, then, this first embodiment'of the present invention resultsin a duct wall sound suppressing treatment including an intermixedplurality of resonant cavities having non-uniform volumes, and aplurality of neck passages providing substantially direct communicationbetween the duct and the cavities. Furthermore, preselected of thecavities are spaced from the duct; and the neck passages are ofnon-uniform length, preselected longer neck passages being associatedwith cavities at relatively greater distances from the duct. The largervolume individuals of the cavities are associated with the longer neckpassages in order to maximize the product of 1 'V, and hence to achieveeffective sound treatment of low frequency sound waves.

The present invention is not, however, limited to this embodiment, sincethe concepts thereof are subject to broad application. For example,referring to FIG. 3, a second embodiment of the present invention isdisclosed. For simplicity, this embodiment is identical with the firstbut for the addition of a porous face sheet 58. The face sheet partiallydefines duct 12 and enhances the aerodynamic efficiency of the duct ascontrasted with the plurality of neck passage openings in FIG. 2. Inthis embodiment, the neck passages 36, 46 and 56 terminate at the facesheet, but the nature of the sheet is such that sound waves freely passtherethrough for effective sound wave entrance to the various cavities30, 40 and 50. In other words, substantially direct communicationbetween the cavity interiors and the duct is maintained despite theapplication of face sheet 58. In operation, the embodiment of FIG. 3performs substantially similarly to that of FIG. 2.

Numerous other variations of the concepts of the present invention willbe apparent to those skilled in the art. For example, theinterrelationship between the cavities disclosed in the precedingembodiments can be substantially varied without varying the overalloperation of the acoustic panel. Furthermore, in order to further expandthe sound frequency dissipating range of the present invention, furthercavities and associated neck passages can be interspersed havingdifferent l'V products and thus, difierent frequency rangecharacteristics. As another variation, the relative volumes of thevarious cavities could be adjusted with similar adjustments to neckpassage lengths while maintaining characteristic frequency suppression.This would allow the panel to be narrowed and elongated or shortened andthickened depending upon appropriate limitations in given applications,due to the equalizing interaction between neck passage length and-volumein the l 'V product. Such variations and those similar to it areintended to be interpreted as falling within the present invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A sound suppressing panel for use in ducts, the panel comprising:

a first plurality of cavities having predetermined first volumes;

a second plurality of cavities having predetermined the first cavitiesby a smaller distance than the sec ond cavities; said first meansincluding a plurality of first neck passages, and said second meansincluding a plurality of second neck passages, the second neck passagesof greater length than the first; wherein the plurality of second neckpassages is interspersed among the plurality of first neck passages; andsaid first cavities include first walls of predetermined first length,and opposed pairs of said first walls of adjacent first cavities, intheir lengthwise dimension, partially define said second neck passages.2. A sound suppressing panel for use in ducts, the panel comprising:

a first plurality of cavities having predetermined first volumes; asecond plurality of cavities having predetermined second volumes; firstmeans for providing substantially direct communication between saidfirst cavities and said duct; and second means for providingsubstantially direct communication between said second cavities and saidduct; said first and second cavities separated from the duct, the firstcavities by a smaller distance than the sec ond cavities; said firstmeans including a plurality of first neck passages, and said secondmeans including a plurality of second neck passages, the second neckpassages of greater length than the first; wherein the plurality ofsecond neck passages is interspersed among the plurality of first neckpassages; and further including a third plurality of cavities havingpredetermined third volumes, the third cavities separated from the duct,and a plurality of third neck passages for providing substantiallydirect communication between the third cavities and the duct; whereinsaid first cavities include first walls of predetermined first length,opposed pairs of said first walls partially defining said second neckpassages; and said second cavities include second walls of predeterminedsecond length, opposed pairs of said second walls partially definingsaid third neck passages.

7 8 3. The panel of claim 2 wherein said second length 5. The panel ofclaim 4 wherein: 18 greater thanl safidlfirst lze gg said first andsecond walls of preselected first and T e pane c mm W erem' secondcavities are common Walls to the extent of preselected pairs of saidfirst cavities are substantially circumscribed by preselectedindividuals of said 5 second cavities.

1. A sound suppressing panel for use in ducts, the panel comprising: afirst plurality of cavities having predetermined first volumes; a secondplurality of cavities having predetermined second volumes; first meansfor providing substantially direct communication between said firstcavities and said duct; second means for providing substantially directcommunication between said second cavities and said duct; said first andsecond cavities separated from the duct, the first cavities by a smallerdistance than the second cavities; said first means including aplurality of first neck passages, and said second means including aplurality of second neck passages, the second neck passages of greaterlength than the first; wherein the plurality of second neck passages isinterspersed among the plurality of first neck passages; and said firstcavities include first walls of predetermined first length, and opposedpairs of said first walls of adjacent first cavities, in theirlengthwise dimension, partially define said second neck passages.
 2. Asound suppressing panel for use in ducts, the panel comprising: a firstplurality of cavities having predetermined first volumes; a sEcondplurality of cavities having predetermined second volumes; first meansfor providing substantially direct communication between said firstcavities and said duct; and second means for providing substantiallydirect communication between said second cavities and said duct; saidfirst and second cavities separated from the duct, the first cavities bya smaller distance than the second cavities; said first means includinga plurality of first neck passages, and said second means including aplurality of second neck passages, the second neck passages of greaterlength than the first; wherein the plurality of second neck passages isinterspersed among the plurality of first neck passages; and furtherincluding a third plurality of cavities having predetermined thirdvolumes, the third cavities separated from the duct, and a plurality ofthird neck passages for providing substantially direct communicationbetween the third cavities and the duct; wherein said first cavitiesinclude first walls of predetermined first length, opposed pairs of saidfirst walls partially defining said second neck passages; and saidsecond cavities include second walls of predetermined second length,opposed pairs of said second walls partially defining said third neckpassages.
 3. The panel of claim 2 wherein said second length is greaterthan said first length.
 4. The panel of claim 2 wherein: preselectedpairs of said first cavities are substantially circumscribed bypreselected individuals of said second cavities.
 5. The panel of claim 4wherein: said first and second walls of preselected first and secondcavities are common walls to the extent of said first length.